1.3 Rocket Ascent Profile and Gravity Turn

[Sunesha]

Thanks a thousand times. I new to the game. Early in career mode. So I have limitations. I spent 3-4 hours trying to build rocket that could orbit and deorbit. I succeed successfully with that but the margin of error was so slim that tried over and over again

However, I read guide when I understood it was more a pilot error than rocket design error. Now I had a huge margin of error after following this guide. Though all my own experiments was a great lesson. 

[Human Person]

Really nice guide, but there is something misleading here:

On 30.6.2015 at 7:10 PM, A_name said:

Aerodynamic Stability: You want your rocket to be aerodynamically stable. That means that it will have a natural tendency to fly straight, instead of, say, sideways. Any object that flies through the atmosphere will naturally orient itself with its center of mass (COM) facing forwards relative to its trajectory and its center of drag (COD) facing backwards. You can see this in darts, arrows, badminton cocks, etc. Similarly, you will want have your rocket's COM in front of your COD. This can be tricky because there isn't a COD indicator in the VAB and because the COM of a rocket shifts during ascent due to shedding weight by burning fuel (Note: as of 1.2 the fuel burns evenly from all tanks within the active stage, so the COM shift will be minimal compared to before). However, you can mostly get away with it if you just add 3 or 4 winglets or wing surfaces with radial symmetry at the base of the rocket. If your rocket insists on flipping, you need to add more/larger wings at the bottom and/or make your payload more aerodynamic by covering it in a fairing. If that doesn't fix it, it means your COM is shifting so much that it falls behind the center of drag when fuel is burnt. The easiest way to fix this is to add a small fuel tank at the top of the stage that's experiencing the problem and lock it in the VAB (right click on the tank and select the green arrows for both fuel and oxidizer). This fuel tank will act as ballast keeping your COM forward. You can unlock it manually in flight when the rest of the stage's fuel is gone so as to not waste it, and then stage as normal. As a final note on aerodynamic stability, you don't want to make your rocket too stable, or else it won't want to turn at all and you won't be able to do a gravity turn. Every rocket will have a different "sweet spot" of stability for the best gravity turn. You need to do test flights, revert, tweak your design until you get it right.

 

 

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Aerodynamic stability will NOT influence your gravity turn at all. That's because "from the rocket's point of view", it flies perfectly straight all the time, just like an arrow's ballistic arc. Many people here seem to think, their rocket will fly "more straight" with a high aerodynamic stability, somehow magically improving their launch profile.

Yes, it is important to avoid your rocket spinning out of control but with a decent gimbal-range and a well balanced (not tail heavy) rocket, you'll be able to fly a perfect gravity turn just as fine as a rocket with a lot of wings at the back. That's why hardly any modern Rocket has fins anymore. However, aerodynamic surfaces can improve the maneuverability of your rocket making course-corrections a lot easier and yes, too many passive aerodynamic surfaces can make it hard to turn.

Edited March 14, 2017 by Physics Student

[A_name]

4 hours ago, Physics Student said:

Really nice guide, but there is something misleading here:

Aerodynamic stability will NOT influence your gravity turn at all. That's because "from the rocket's point of view", it flies perfectly straight all the time, just like an arrow's ballistic arc. Many people here seem to think, their rocket will fly "more straight" with a high aerodynamic stability, somehow magically improving their launch profile.

Yes, it is important to avoid your rocket spinning out of control but with a decent gimbal-range and a well balanced (not tail heavy) rocket, you'll be able to fly a perfect gravity turn just as fine as a rocket with a lot of wings at the back. That's why hardly any modern Rocket has fins anymore. However, aerodynamic surfaces can improve the maneuverability of your rocket making course-corrections a lot easier and yes, too many passive aerodynamic surfaces can make it hard to turn.

Yes, I see your point, although I don't see how the OP is misleading, could you clarify?

[Human Person]

 

@A_name I quoted the wrong part there, i meant this one

On 30.6.2015 at 7:10 PM, A_name said:

2. As soon as you hit 50 m/s, perform a pitch-over maneuver to begin your gravity turn. To do this, tip your rocket towards the East slightly, until it is pointing between 5° to 10°. The higher your thrust and the more aerodynamically stable your rocket is, the more you need to pitch over initially. Remember, higher thrust makes the rocket want to go straight, as does aerodynamic stability. Don't start pitching over before your speed is ~50 m/s, otherwise you will likely find yourself horizontal within a few seconds, as your winglets won't be biting into the air hard enough to provide stability.

 

 

 

 

 

 

2

 

I highlighted the part. What you say isn't wrong, just misleading. It should be clarified.

You mention high TWR and aerodynamic stability in one sentence, as a factor to "make the rocket fly straight". By that, you mean two completely different things.

High TWR influences the shape of your ballistic arc, thereby it slows down the rate of turn your rocket experiences whereas aerodynamic stability just keeps your rocket pointed prograde (pointed in the direction of the ballistic arc).

As I read in some comments, people seem to think that more aerodynamic stability influences their path of flight. E.g.:

On 30.12.2015 at 0:19 PM, rodentgun said:

My rocket appears to be aerodynamically very stable, but at this speed it drops into the horizon too quick.

 

 

 

 

 

 

If you wanted this to be the case, you would need an Angle of Attack for the fins to create lift.

 

Edit: I'd only recommend using fins for small rockets, rockets with very large (un-aerodynamic) payloads and rockets with non-gimbaling engines on the first stage. Especially the last case has to have active aerodynamic control surfaces.

Edited March 14, 2017 by Physics Student

[A_name]

33 minutes ago, Physics Student said:

 I quoted the wrong part there, i meant this one

Thanks, good catch. I'll edit it.

[Human Person]

Again, this tutorial is very well put togethe and I'm sure it would be a lot worse if i had written it.

 I only spotted this minor error because it was one of my own mayor rookie false assumptions when building my first rockets. I actually thought I could make them fly better by putting more fins on. Now I save that space for more boosters.

Edited March 15, 2017 by Physics Student

[MR L A]

Someone should do a video of this and explain whats going on I think S.M. posted one or two but nothing within the last year.

I was trying this method with a small 2.5m design (similar to the stock GPLV - if that's what its called) but it didn't quite work.. after jettisoning the SRBs the vehicle's TWR is momentarily 1.0 (possibly below) resulting in too much turn as you'd expect. Will work fine with smaller payloads, but I was really hoping this particular craft could efficiently deliver this particular payload   

[Human Person]

56 minutes ago, MR L A said:

I was trying this method with a small 2.5m design (similar to the stock GPLV - if that's what its called) but it didn't quite work.. after jettisoning the SRBs the vehicle's TWR is momentarily 1.0 (possibly below) resulting in too much turn as you'd expect. Will work fine with smaller payloads, but I was really hoping this particular craft could efficiently deliver this particular payload   

Those are my favorite designes, they are quite cost efficient but challenging to fly. Usually I build them as an SSTO with boosters. They require a higher trajectory and a very gentle gravity turn. Sometimes it helps reducing the thrust on the SRBs a bit. This reduces your TWR at launch, but your boosters burn longer, reducing the drop in TWR at booster separation.

Edited April 18, 2017 by Physics Student

[MR L A]

1 minute ago, Physics Student said:

Those are my favorite designes, they are quite cost efficient but challenging to fly. They require a higher trajectory and a very gentle gravity turn. Sometimes it helps reducing the thrust on the SRBs a bit. This reduces your TWR at launch, but your boosters burn longer, reducing the drop in TWR at booster separation.

I was thinking that might be a solution

I agree, they're more fun to fly. Actually have to think about what you're doing a little more whereas the bigger rockets (in the same series as this smaller one) have ample TWR throughout the flight. But tbf, the missions with the big rockets tend to be a little more exciting than crew rotation of a space station in a 1000km orbit... 

[Exoscientist]

 Can this be used to launch to low Earth orbit, not just low Kerbin orbit?

  Bob Clark

[Ferdoni]

On 30-6-2015 at 7:10 PM, A_name said:

 

1. Turn on SAS and set your throttle to whatever will give you a TWR of ~1.5.

 

This could be very higher for more efficiency

[MoridinUK]

I'm having trouble with this but I guess it is down to having my first 2k of delta v locked in four big solid fuel boosters! I just cant get the turn right, basically sas is never in the middle ready to be turned off... 

The rocket is huge and has tonnes of Dv 4.4k and so should reach orbit much more efficently than I am pulling off...  

I'm going to try it in a 3.2/3.5k dv craft using only liquid fuel and see what happens...

 

[A_name]

Sorry for the late reply guys, I've been taking a break the last couple months. I'll post anyway for posterity.

 

On 24/4/2017 at 3:17 PM, Ferdoni said:

This could be very higher for more efficiency

 

Indeed, but with drawbacks on stability, aerodynamics, and heating. An initial TWR of 1.5 is in my experience the sweet spot between efficiency on one hand, and on the other hand being able to perform the pitch-over fast enough and keeping a reasonable turn rate that won't require manual steering nor have you going horizontal in the thick atmosphere only to burn up or suffer massive drag losses.

 

On 17/5/2017 at 5:18 PM, MoridinUK said:

I'm having trouble with this but I guess it is down to having my first 2k of delta v locked in four big solid fuel boosters! I just cant get the turn right, basically sas is never in the middle ready to be turned off... 

The rocket is huge and has tonnes of Dv 4.4k and so should reach orbit much more efficently than I am pulling off...  

I'm going to try it in a 3.2/3.5k dv craft using only liquid fuel and see what happens...

 

It's definitely tricky with SRB's due to their high thrust and inability to throttle down, which leaves you with little control over what your launch will look like after the first few seconds.

[A_name]

Updated.

[xionix4]

On 1/2/2016 at 12:32 PM, rodentgun said:

Ah, thank you pvtsteyr. I have only played KSP since december so the science and fine art of ballistics is new to me. Learned what I have from this I changed the way how I launch Spaceplanes too. Instead of sharp pitching up I gently raise the nose up above Kerbin's horizon when my jet hits max thrust at 11 km. By slowly nudging the prograde vector upwards until I hit 35 degrees I have now A good 20% efficiency boost at least. More deltaVs!

 

anyways, what IS the necessary velocity to reach orbit around Kerbin? something above 2200 m/s?

I know you asked this a while ago, @rodentgun, but I hope you're still playing KSP.

Either way, you can calculate circular orbit speed for any celestial body using the formula sqrt(mu/r).

Sqrt is the square-root function, mu (Greek letter mu, also known as the Standard Gravitational Parameter) is found in map view / tracking station in the info window (listed as GM, because it's the product of the Universal Gravitational Constant, G and the mass of the celestial body, M), and r is the radius you want to orbit at (NOT the altitude -- you get r by adding the desired altitude to the radius of the celestial body, also listed in the info window of map view / tracking station). This formula works for real celestial bodies, too.

To answer your question directly, Kerbin has a mu (GM) of 3.532x10^12 m^3/s^2 and a radius of 600 km. Since the Karman line (where the atmosphere is considered to end, and you are in space above it) is 70 km for Kerbin (listed as Atmos. Height in map view / tracking station), orbit radius is anything above 600 km planet radius + 70 km altitude = 670 km = r.

Plugging into the formula, we get sqrt([3.532x10^12 m^3/s^2] / [670,000 m]) = 2,296 m/s at 70 km altitude.

You may have noticed that as r increases, the result becomes smaller, and indeed, it requires less speed to orbit at higher altitudes. (I like to shoot for 80 km myself, which has a circular orbit speed of 2279 m/s).

As you know, we of course need more delta-v than that to get up there and to overcome losses during the ascent, but I hope this answered your question.